Primary and secondary batteries such as lithium battery, lithium ion battery and lithium polymer battery using an alkali metal such as lithium or an alloy or compound thereof as an anode active material are attracting attention in recent years because of their large capacities. The cathode material for use in such primary and secondary batteries is subjected to electrode oxidation/reduction accompanied by doping/undoping of ions of an alkali metal such as lithium during the process of charging and discharging. As the cathode material, the ferric phosphate (FePO4) having a trigonal P321 crystalline structure is well-known in the prior art (Japanese Patent No. 3126007).
Although Japanese Patent No. 3126007 discloses a method for obtaining ferric phosphate anhydride by heat treatment of ferric phosphate hydrate (FePO4.nH2O), there in no description of the method for synthesizing ferric phosphate hydrate.
An example in which a FePO4 cathode active material having a trigonal P321 structure was synthesized from NH4H2PO4 and Fe(NH4)2(SO4)2.6H2O at a temperature of 650° C. has been reported [P. P. Prosini et al, J. Electrochem. Soc., 140, A297 (2002)] has been reported. However, the material has as low a capacity as 40 mAh/g.
Conventionally, ferric phosphate hydrate as a calcination precursor is synthesized by, for example, mixing a solution containing a tervalent iron such as a solution of iron (III) sulfate or ferric chloride (or a hydrate thereof) with an alkaline compound containing phosphate ions such as disodium hydrogenphosphate, allowing the reaction mixture to stand under an elevated temperature, and filtering the precipitate. The ferric phosphate hydrate, however, is not suitable for a material of cathode for a secondary battery because nonvolatile elements such as sodium ions tend to remain as impurities. That is, in the synthesis method, it is necessary to remove sodium ions and so on from the calcination precursor by filtering. The process is cumbersome and may bring the entry of impurities. To complete the filtering and increase the purity of the calcination precursor, it is preferred to allow the crystals of ferric phosphate hydrate precipitate to grow until they reach a large diameter (about 10 μm or greater). However, when ferric phosphate hydrate particles having a large diameter are calcined, the resulting ferric phosphate particles have a large diameter and have low activity as a cathode material. It is known that the performance of a cathode material is largely affected by the size, shape and specific surface area of the particles thereof and impurities therein.
It is, therefore, an object of the present invention to provide a method for producing a cathode material by which ferric phosphate suitable as a cathode material for lithium batteries such as lithium primary and secondary batteries can be synthesized reliably and easily, and to provide a high-performance lithium battery primary or secondary battery) using the cathode material obtained by the method.